6. Convection

Convection refers to any transfer of thermal energy by motion of a medium.

The rate equation or the convective heat transfer (regardless of particular nature) between a surface and an adjacent fluid is prescribed by Newton’s law of cooling. \[\boxed{Q=hA(T_s-T_\infty)}\] where

\(Q\) = Rate of heat transfer (W)
\(A\) = Area exposed to heat transfer (m\(^2\))
\(T_s\) = Surface temperature of solid (\(^\circ\)C)
\(T_\infty\) = Fluid temperature (\(^\circ\)C), and
\(h\) = Heat transfer coefficient (W/m\(^2\).\(^\circ\)C)

\[\boxed{h = f (\text{geometry, fluid motion, fluid properties, $\Delta T$})}\]


Forced vs. Natural:

  • Forced convection  refers to the case when the fluid is made to flow by some external agent, using a pump, fan, stirrer, etc.

  • Natural convection refers to fluid motion which naturally occurs from the heat transfer itself, due to buoyancy differences (“hot air rises...”).





Typical values of heat transfer coefficient [W/(m\(^2\).)]:
Free convection gases: 2 –­ 25
liquids: 50 – 100
Forced convection gases: 25 –­ 250
liquids: 50 –­ 20,000
Boiling/Condensation 2500 – ­100,000






Boiling water has a higher heat transfer coefficient than subcooled water.